A Push-to-talk over Cellular (PoC) system using communication terminals, such as cell phones, has been put into practical use. In the PoC system, one user having a communication terminal can send a voice to the communication terminals of the other users at once. In other words, the PoC system can perform one-to-many communication and thus the PoC system has been received considerable attention as a simple communication tool in a group of a plurality of users. In this system, the user has to push a talk button on the communication terminal to acquire the right to talk, followed by talking. Voice that correspond to the talking during a period from the push of the transmission button to the abandonment (release) of the right to talk are simultaneously sent to other communication terminals, so that other users can hear the voice.
In the PoC system, however, when one user is talking, other users are not allowed to acquire the right to talk. Thus, two or more users cannot perform voice transmission at the same time.
Furthermore, the PoC system includes a server for managing the right to talk and sending/receiving audio data. In response to the request of the right to talk from a communication terminal, the server provides the communication terminal with the right to talk to permit this terminal to talk. Subsequently, the server receives audio data sent from the talking-permitted communication terminal and then broadcasts the audio data to other communication terminals. In this case, the server is set to broadcast audio data to communication terminals included in a previously registered communication group. The server manages such a communication group.